Conventional electric motors in general make use of electro-magnetic effects and derive driving force or moment from the interaction of electric current and magnetic field, but a new kind of electric motor utilizing electrostrictive effect has been developed recently. It is generally known that many piezoelectric elements exhibit a reverse effect. The electrostrictive effect and the prospect of developing a new electric motor based on the electrostrictive effect is very attractive, because these electric motors, which are entirely different from the electromagnetic motors, have many advantages such as fast response, ease of control, high torque at low speeds, and so are especially suitable for automatic control robotic applications.
Some achievements of research on piezoelectric electric motors has already been published in U.S. Pat. No. 4,513,219 and 4,399,386. In these patents, two kind of rotary electric motors using piezoelectric elements are described. The basic structural peculiarity of these motor lies in the use of a plurality of equally spaced piezoelectric elements circumferentially disposed. When ac or pulsed dc voltage is applied sequentially to these elements, following a certain pattern, the piezoelectric elements will oscillate in a curvilinear orbit, and the output rotor will then be driven continuously by the oscillatory motion of the piezoelectric elements through the frictional contact of the rotor with the piezoelectric elements.
But the above mentioned mechanisms are plagued by the following shortcomings:
1. The oscillatory motion of the piezoelectric elements is not compatible with the continuous uniform motion of the output rotor. Effective motion transfer occurs only during less than one half of the oscillatory cycle, resulting in low efficiency;
2. Creep between the piezoelectric elements and the friction material of the output rotor will cause power loss and wear;
3. When the mechanism is overloaded or reverse drive takes place by external load, serious wear or even failure will occur;
4. Because of the presence of creep between the piezoelectric elements and the output rotor, the output speed is dependent on the external load;
5. High speed operation is unattainable;
6. The arrangements of such prior art patents can only be adapted to miniature motors, high power application is infeasible